In communication systems designs, analog filters are usually required for image removal/rejection or anti-aliasing. Analog filters are the family of filters that are capable of taking analog signals and outputting analog, continuous-time or discrete-time signals. It is desirable to have analog filters integrated onto an integrated circuit to reduce the overall cost and the size of the system. Two types of filter are commonly used in integrated circuit applications: the switched capacitor (SC) filter and the transconductance-capacitor (gm-C) filter. The SC filters have very precisely defined filtering characteristics because the time constants associated with the frequency response depend only on the capacitor ratios and the clock frequency. SC filters are known to have a number of drawbacks. First, when used at a typical intermediate frequency (IF) in a radio system, it is capable of causing aliasing of interfering signals. Secondly, it has a noise problem which can only be mitigated at the price of increased power consumption. On the other hand, gm-C filters typically provide lower noise and consume less power than SC filters but suffer in the area where SC filters perform well. Since the time constant in a gm-C filter are determined by gm and C, the frequency response is sensitive to process variations, temperature drift and power supply variations. Calibration loops are required to cancel out these effects but complicates the designs significantly. For these reasons, it is difficult to construct very high-order high-precision gm-C type of filters. Both of these filter types have difficulties producing area-efficient FIR filters.
Recently, another type of analog filter has been attempted. It involves generating one current or multiple currents from an input voltage signal, selectively integrating the current(s) to a pair of capacitors (as for example in U.S. Pat. No. 6,829,311) or multiple capacitors for a predetermined time interval, and then sampling the charges accumulated on the capacitors and then resetting for the next current integrating cycle. The selective current integration operation realizes a finite impulse response (FIR) filter. In one reference, multiple currents are generated to be selectively integrated on a capacitor. The currents are generated in such a way that the currents are proportional to the input signal and the tap coefficients of a desired FIR filter. The current generation is realized by having multiple transconductance amplifiers with gains corresponding to the tap coefficients. Since transconductance of the amplifiers are dependent upon process variations, temperature drifts and power supply variations, the resultant filter frequency response is sensitive to these variations as in the case of the conventional analog filters. In U.S. Pat. No. 6,829,311, a single current converted from the input voltage signal is sent to a pair of capacitors. The FIR filter taught by this reference substantially reduces the above sensitivity problem as its frequency response depends only on the clock frequency and matching accuracy of the current integration capacitors. However, it is limited to filters with tap coefficients comprising 0's and 1's which prevent it from realizing arbitrary FIR filters. The limitation can be worked around by quantizing an arbitrary set of tap coefficients using a delta sigma technique. Since there are only two quantization levels, 0 or 1, the resulting quantization error in the filter frequency response can be very high which requires additional filtering to reduce it to acceptable levels.
It should also be noted that sampling filters in the prior art have shortcomings that reduce their effectiveness when applied to real-world applications. Prior art sampling filters do not disclose filters with an impulse response longer than the period of the output sampling frequency. This does not allow for high sampling frequencies for narrow band filters that require long impulse responses.
Another drawback of present sampling filters is that they require extra filtering to remove quantization error in the tap coefficients when the tap coefficients are Delta-Sigma quantized. They are also excessively sensitive to analog mismatches in the tap currents.
Finally, in a sampling filter, the tap current at any given current integrating circuit increases from zero to the final value for every given predetermined interval. However, this introduces distortions to the filter transfer function or noise because of the tap current switching transients.
A need therefore exists for an analog filter with well controlled frequency response but with minimum quantization error in the transfer function. Furthermore, there is a need for methods and devices that avoid or mitigate the shortcomings of the prior art.